DEV: allows categories-admin-dropdown to have custom items (PR #9931)

Usage:

modifySelectKit("categories-admin-dropdown").appendContent(function() {
  return {
    id: "foo",
    name: I18n.t("foo"),
    description: I18n.t("foo_description"),
    icon: "gear",
    onChange: () => alert("Foo.")
  };
});

GitHub